In four-cycle engines, oil is fed to engine components located therein to lubricate and cool them. These engines are provided with oil passages and the oil is pumped by an oil pump configured to rotate in cooperation with a crankshaft to flow to the engine components through the oil passages. The crankshaft is accommodated in a crankcase. The oil is fed to a region at which a crank journal and a journal bearing are configured to slidably contact each other or to a region at which a crank pin and a big end of a connecting rod are configured to slidably contact each other, in order to lubricate and cool them.
Japanese Utility Model Application Publication No. Sho. 61-58730 discloses a configuration to feed oil to the above regions of the crankshaft. According to this configuration, an oblique oil passage is formed to extend obliquely with respect to a rotational axis of the crankshaft and to couple a center axis of a crank journal to a center axis of a crank pin (see FIG. 2). To be specific, the oblique oil passage extends from the crank journal to the crank pin through a crank web and further penetrates through another crank web and is configured to open at the latter crank web. A blank plug is attached to this opening (see FIG. 1).
Further, the crank journal is provided with a first oil passage configured to extend radially and to penetrate through the crank journal. The first oil passage is connected to the oblique oil passage. The crank pin is provided with a second oil passage configured to extend radially and to penetrate through the crank pin. The second passage is also connected to the oblique oil passage.
In engines equipped with the above constructed crankshaft, the oil is fed to a groove formed in the journal bearing for the crank journal. Some oil is fed to a region at which the journal bearing and the crank journal are configured to slidably contact each other, and the remaining oil flows into the interior of the crank journal through the first oil passage and further toward the crank pin through the oblique oil passage. Further, the oil is delivered to an outer peripheral surface of the crank pin through the second oil passage in the interior of the crank pin and is fed to a region at which the crank pin and the big end of the connecting rod are configured to slidably contact each other.
The oil is pumped by the oil pump configured to rotate in cooperation with the crankshaft to be fed to the engine components. The pressure of the oil is generally determined by a capacity of the oil pump and an engine speed of the engine. Regarding the oil fed to the crank pin through the oil passage in the interior of the crankshaft, a desired pressure of the oil increases in proportion to the square of the engine speed, i.e., the number of rotations of the crankshaft.
In the case of the crankshaft provided with the oblique oil passage, and the first and second oil passages, the oil flows through the first oil passage and then is delivered to flow radially inward from an outer peripheral region of the crank journal to the center axis of the crank journal where the first oil passage is connected to the oblique oil passage. Therefore, during rotation of the crankshaft, the oil flowing in the first oil passage is subjected to a centrifugal force applied in an opposite direction to a flow direction of the oil. This centrifugal force increases with an increase in the number of rotations of the crankshaft and impedes the flow of the oil.
In order to suitably feed the oil to the engine components during high-speed rotation in the engine disclosed in the above Japanese Utility Model Application Publication No. Sho. 61-58730, it may be necessary to equip a larger oil pump to ensure a higher pressure of the oil. The larger oil pump causes a larger friction loss of the associated components.
Furthermore, in the configuration disclosed in the above Japanese Utility Model Application Publication No. Sho. 61-58730, the oblique oil passage, and the first and second oil passages extend in different directions. During drilling bores in the crankshaft to form these passages, it is necessary to change orientation of a drill machine or change orientation of the crankshaft and to re-fix the crankshaft, resulting in a burdensome manufacturing process.